See — Temperature of the Sun and Ages of Stars and Nebulae. 41 



(f.) Temperatures of the Great Planets. As experiments 

 upon the secular shrinkage of great masses cannot be made 

 in our laboratories, it is fortunate that the solar system offers 

 to our observation large as well as small planets which may 

 be taken to be approximately of the same absolute age. We 

 find the smaller planets such as the Earth, Venus, Mars, and 

 Mercury, already solid, while the great planets Jupiter, Sat- 

 urn, Uranus, and Neptune, are apparently still gaseous, if not 

 actually rising in temperature. A similar comparison holds 

 for the Moon and Jupiter's satellites which are much more 

 advanced in their development than the planets about which 

 they revolve. The law of temperature shows that if bodies 

 like Jupiter and Saturn are gaseous, they have not been hot 

 in the past, but may become so hereafter. There is some 

 spectral indication of inherent luminosity in Uranus, and 

 hence all the great planets are probably still rising in tem- 

 perature. As the temperatures of these masses were origi- 

 nally near the absolute zero of space, we are not to think of 

 them as cooling, but rather as having slowly heated up ever 

 since their separation from the solar nebula. 



The inferences of Kant, Zollner, and Proctor, as well as 

 the original assumption of Laplace, all implying an initial 

 high temperature, it is needless to say, are wholly unauthor- 

 ized. It is possible and perhaps even probable, that some of 

 the great planets, especially Jupiter and Saturn, may eventu- 

 ally become self-luminous. 



The problem as to how closely the purely gaseous theory 

 conforms to the actual state of the heavenly bodies is very 

 important, but unfortunately difficult to answer with confi- 

 dence. On the one hand, the purely gaseous theory leads to 

 a height of 27.5 Kilometres for the terrestrial atmosphere ; on 

 the other, observations of meteors, which disclose the fact 

 without regard to theory, show that it extends in a rarified 

 state to a height of at least 200 Kilometres. From this well- 

 established deviation of theory from phenomena, it would 

 appear that the purely gaseous atmosphere extends to its 

 proper height, and is then overlaid by another layer in the 

 ultragaseous state. Presumably this upper ultragaseous 

 atmosphere is one in which the molecules have a long free 



